Introduction to Data Communications
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15. Voice Channel Communications

The voice channel or dial-up line is the line from our telephone/modem to the outside world.

As the name implies "voice" channel is designed to carry human speech over the telephone wires.

15a. Voice Channel Specification

Human speech covers the frequency range of 100 to 7000 Hz (hertz) but research has shown that the intelligence part of human speech is carried in the 300 - 3400 Hz range. This range is called the Voice Band.

The Voice Channel has a range of 0 to 4 kHz (4000 Hz). The area from 3400 to 4000 Hz is used for system control and is called Out of Band Signalling.

15b. Voice Channel Constraints

Due to the limited Bandwidth (BW) of the Voice Channel (0-4 kHz), we are limited to the amount of data that we can pass through the Voice Channel. The Nyquist Theorem addresses this limitation.

Nyquist Theorem

In a digital Public phone system, the signal leaving our telephone at our house is an analog signal. It goes to the Central Office through the Local Loop. The Local Loop is the name for the wires that run from our house to the Central Office. The Central Office (also called a local exchange) is the building that all the neighbourhood phones with the same local connect. A local is the 1st 3 digits of your 7 digit phone number or LDN (Listed Directory Number).

At the Central Office, the analog signal is converted into a digital signal consisting of 1s and 0s.

The Nyquist Theorem states that to accurately reproduce an analog signal with a digital signal, the analog signal must be sampled a minimum of 2x the highest frequency of the analog signal.

This means that for the Voice Channel (0 to 4 kHz) to be digitized, we must sample the Voice Channel at 2x the highest frequency (4 kHz) which would be 8 kHz. This means that as soon as you digitize an analog signal, you must immediately double the bandwidth.

Introduction to Data Communications
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Copyright Eugene Blanchard Jan 1998, January 2007